Occupant detection and classification devices, which are based on optical sensors are now available in modern cars. The information determined by such devices is a seat occupancy class, which is often used for safety critical applications such as smart control of adaptive air bags. These devices typically consist of a control and computing unit and a sensor unit, where the control unit analyzes the images provided by the sensor unit in order to determine the most probable seat occupancy class, for instance “empty”, “child seat” or “passenger”, and afterwards forward it to the air bag control unit.
Conventional optical detection and classification devices typically rely on a real-time range camera without moving parts based on the time-of-flight (TOF) principle (an example of such TOF range camera is e.g. described by R. Lange and P. Seitz in “Solid-State Time-of-Flight Range Camera”, IEEE Journal of Quantum Electronics, Vol. 37, No. 3, March 2001). The detection and classification device mainly comprises a computing unit and a sensor unit. The sensor unit features an imager sensor operated on the TOF principle and an associated optical interface comprising one or more lenses to collect and focus the light coming from the observed scene onto the imager. The sensor unit also integrates the active illumination module emitting e.g. visible or near infrared radiation that will reflect on the objects in the scene and be detected by the sensor unit.
An inherent problem of any optical system is that of contamination (un-cleanliness). Indeed, the optical interface of the sensor unit to the car interior in an occupant detection and classification device is subject to contamination such as dirt, dust, dew, grease, scratches, finger tips etc. coming from the passenger area. By modifying either locally or homogeneously the optical properties of the sensor unit, contamination has the possible effect to modify the performance of the operation of the control and computing unit and therefore to possibly introduce errors in the determined information, for instance the occupancy class of an occupant detection and classification device.
The problem of contamination is particularly acute for such safety critical applications and it would be good to have a simple and efficient method of determining contamination in such TOF range camera for this and other types of applications.